More than 16,000 residents were evacuated as volcanic activity on La Soufrière increased on Thursday, with the volcano emitting plumes of ash.
Evacuees are being given temporary homes on cruise ships and in safer parts of the island.
It is the first volcanic eruption on Saint Vincent since 1979 - the worst one on record killed over 1,000 people in 1902.
As darkness fell on Thursday, a lava dome became visible on La Soufrière, suggesting it was about to erupt.
Just before 09:00 on Friday (13:00 GMT), seismologists from the University of the West Indies confirmed that an "explosive eruption" was under way.